1. Field of the Invention
The present invention relates to a strip line filter including a dielectric substrate and strip lines arranged on the dielectric substrate.
2. Description of the Related Art
Strip line filters including strip-line resonators arranged on dielectric substrates have been used in various fields (refer to Japanese Unexamined Patent Application Publication No. 7-312503).
Here, an example of an equivalent circuit of a conventional strip line filter will be described. FIG. 1 is a diagram illustrating an equivalent circuit of a conventional microstrip line filter 101 shown with reference to Japanese Unexamined Patent Application Publication No. 7-312503.
The microstrip line filter 101 is configured such that ¼ wavelength strip line resonators in two stages are coupled with each other in a comb-line manner and the ¼ wavelength strip line resonators are externally coupled with respective input/output terminals through external coupling capacitances C01. Each of the ¼ wavelength strip line resonators has a stepped impedance configuration in which lines having different widths are arranged on an open-end side and a short-circuit side, and mutual capacitance on the open-end side and mutual capacitance on the short-circuit side are changed by controlling the line width on the open-end side and the line width on the short-circuit side to thereby control coupling between the resonators.
In order to realize wide-band filter characteristics in the conventional strip line filter, the line widths on the open-end sides should be made smaller so that the coupling between the resonators is enhanced. However, accuracy of shapes of electrodes is limited, and when the line widths are made smaller, variation of the characteristics due to size variation increases. Accordingly, a possible setting value of mutual capacitance has an upper limit and enhancement of the coupling between the resonators also has limit.